1. Field of the Invention
This invention relates to the field of semiconductor manufacture and, in particular, to an in-line method of monitoring die attach material adhesive weight.
2. Description of the Related Art
Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps. For each wafer produced, this process yields a number of die on the wafer. The die are separated with a die saw, and then packaged into individual components.
During the manufacturing process, several die are attached to a support element, often with a material such as epoxy or other viscous adhesives. The support element can be metal, ceramic, organic or any other support element used in electronic applications. A typical example of such a support element is a lead frame. For purposes of illustration only, a lead frame will be used as the support element. Wires are connected from the bond pads on each of the die to the "fingers" on the lead frame, then the die is encapsulated in a protective plastic casing. The plastic packages are separated, and the leads are formed into a desired shape. These leads couple the die with the device into which the component is installed, thereby forming an input and output means between the die and the device.
The lead frame, part of which will eventually form the conductive leads of the components, contains a major surface to which the die is attached, called the "paddle." The die is normally bonded to the paddle with epoxy or another viscous adhesive, although thermoplastic, tape, or another materials are also used. FIG. 1 shows a lead frame 10 having die paddles 12 with die 14 attached. The adhesive used to attach the die 14 to the lead frame 10 is dispensed on the die paddle 12 area of the lead frame 10. The die 14 is then placed on the die paddle 12. The die 14 is placed on the uncured epoxy and held at a specific pressure by die attachment equipment having a surface contact tool or an edge contact only tool (collet). The die 14 is pressed down into the adhesive at a specific pressure by the tool and held in place long enough to ensure adhesion. The attach process requires a follow-on cure in a separate cure oven. FIG. 2 shows a die 14 properly attached to a lead frame 10 by an adhesive 20. FIG. 2 also illustrates one example of how the bond pads 15 are subsequently bonded to the lead frame 10 by wires 30 to form an electrical connection between the die 14 and the leads of the lead frame 10.
Many problems result from a nonuniformity in the thickness (the "bond line") or amount of the adhesive 20 bonding the die 14 to the paddle 12. The adhesive 20 is generally applied to the paddle 12 by an adhesive dispenser. Most dispensers use a short pulse of air (usually lasting a few milliseconds) maintained at a predefined pressure to apply the adhesive 20 to the paddle 12. Accordingly, one way to control the amount of adhesive 20 applied is by increasing or decreasing the length of the pulses. Longer pulse lengths lead to more adhesive 20 while shorter pulse lengths lead to less adhesive 20 being applied to the paddle 12. Another way to control the amount of adhesive 20 applied is by increasing or decreasing the pressure used to dispense the adhesive 20. Increasing the pressure leads to more adhesive 20 while decreasing the pressure leads to less adhesive 20 being dispensed from the dispenser.
If not enough adhesive 20 is applied to the die paddle 12 a good bond between the die 14 and lead frame 10 cannot be ensured because of voids in the adhesive 20 under the die 14. If too much adhesive 20 is applied, too much of the adhesive 20 will ooze out from under the die 14 (resin bleed) which can prevent a good coupling between the bond wire and the bond pad during the wire bonding process. Too much adhesive 20 can also lead to a contamination of the lead frame 10 as well as other manufacturing problems.
Since the weight of the adhesive 20 directly relates to the amount of adhesive 20 applied to the lead frame 10, one method of ensuring the uniformity of the amount of adhesive 20 applied to the lead frame 10 is to monitor the adhesive's 20 weight. To do so, the lead frame 10 is weighed prior to applying the adhesive 20 and then weighed again once the adhesive 20 has been applied to the paddles 12. This second measurement requires that the lead frame 10 to be removed from the assembly line which slows down the throughput of the manufacturing process. The difference in the two measurements is then compared to a predetermined desired weight range. If the calculated adhesive weight falls within the range, then a proper amount of adhesive 20 is being applied to the paddle 12. If the calculated adhesive weight falls outside of the range, then an improper amount of adhesive 20 is being applied to the paddle 12.
Although the above technique can properly detect when an improper amount of adhesive 20 is being applied to the paddles 12 of a lead frame 10, it still has some shortcomings. One problem is that the weight measurements, adhesive weight calculation and dispenser control adjustments (if required) are all performed manually by an operator. Accordingly, there is the potential for human error in any one or all of these steps. In addition, the manual monitoring process requires the operator to slow down the throughput of the manufacturing process since the lead frames 10 must be removed from the assembly line to measure the weight of the applied adhesive 20. Since the manual monitoring process slows down the assembly line throughput, the monitoring of the adhesive weight will not be performed often. Accordingly, more lead frames 10 with inappropriate amounts of adhesive 20 will be produced before any necessary adjustments are made to the dispenser.
Therefore, there is a need and desire for a method and apparatus for monitoring the die attach material adhesive weight that can be performed in-line (that is, without removing the lead frame 10 from the assembly line) and without the need for human intervention. In addition, there is a need and desire for a method and apparatus for monitoring the die attach material adhesive weight that can automatically adjust an adhesive dispenser to dispense an appropriate amount of adhesive.